Marsh Fritillaries return to Fullbrook!

Having briefly mentioned that we found marsh fritillary webs on Rhos Fullbrook last month I (Em Foot) thought I’d expand a bit on our surveys and the butterflies themselves.

Every year marsh fritillary larval web surveys are completed on several sites in Ceredigion including some Wildlife Trust reserves. In the past Rhos Pil Bach has been the strong hold for our reserves, with some having been recorded on Rhos Glyn yr Helyg and in the 80s and 90s some on Rhos Fullbrook. Rhos Glandenys has never had any recorded on it despite having plenty of Devil’s bit scabious, the larval food plant.

Surveying using Butterfly Conservation’s methodology (a full sweep method has been used in the past) involves walking transects across likely habitat looking for and counting the larval webs seen within one metre of the line on either side. Using this and the area of the site/suitable habitat the likely number of webs can be calculated and compared with other sites and years.

Marsh Fritillary by M Clark

Marsh Fritillary by M Clark

A web looks a bit like a slimey spider’s web and is generally found at the bottom of the plant mostly on dead leaves. They can be very difficult to spot as they can be underneath the leaves but you soon get your eye in and recognise likely and unlikely places to find them. Within the web are a number (large webs can contain over 100) of little blackish-brown caterpillars/larvae. Each web or group of webs is recorded on a map and management and habitat notes made.

For the last few years numbers at Rhos Pil Bach have plummeted from over 1000 in 2006 and remained very low with only 1 or 2 webs being recorded. Numbers at Rhos Glyn yr Helyg have also decreased with none being found for the last few years, including this year. After hearing that adults were seen at Cors Caron last year I decided that a survey should be completed at our Rhos Fullbrook reserve which is a short distance away.

Feeling not particularly hopeful (no records since 1996 and last surveyed in 2009) we started our survey near the main entrance and zig-zagged our way across the two wet meadows following Butterfly Conservation’s methodology. There was plenty of devil’s bit scabious, the larval food plant, around but I was still very surprised to find a web with larvae in near the far end of the first field on a slightly raised area. Feeling buoyed by our success we continued but found no more webs in that field. With hope fading we started the second field and before long had found another web! The first of many as it turned out. All together we found 20 occupied webs and 2 empty ones! Not bad for a site that hasn’t had them recorded on for 15 years (1996).

Marsh fritillary butterflies lay their eggs on Devil’s bit scabious in June and July. The male holding territory over a single plant or small patch, waiting for the female to fly past. Neither will fly far from their emergence site. They mate and the female lays her eggs on the scabious. The eggs hatch and the small brown larvae build webs on the scabious leaves. This is what we survey for. They over winter as larvae, now much bigger and blacker, at the bottom of the scabious. In the spring they form pupae and the adults emerge in about June. They are fairly weak fliers with hedges often being an insurmountable barrier to them.

The British Isles are thought to be a strong hold for the marsh fritillary within Europe but even here numbers are declining. It is a priority species for conservation and within Ceredigion is a Local Biodiversity Action Plan species. It is found on several other sites in west Wales. Habitat degradation is a major problem and the lack of neighbouring good habitat and populations to allow the species to spread or recover.

We’ll be watching out for adults next summer and will again survey for larval webs next year, we may even survey at Rhos Glandenys having found a suspicious looking web while we were there cutting rushes!